1. Filed of the Invention
The present invention relates to a novel polymeric material which can be made reusable by decomposition and resynthesis. The present invention relates also to a novel process for decomposition-repolymerization of the polymeric material, and to a novel system of material circulation.
2. Related Background Art
Hitherto, various novel materials useful for living and industry have been developed sucessively by coal chemical techniques and by petrolchemical techinques. Typical examples include plastic materials such as polyethylene, polypropylene, polystyrene and polyvinyl chloride; and rubbers such as polyisoprene and polybutadiene. In recent years, resin materials having unique properties have been developed such as polyimide resins having excellent heat resistance and high impact strength, and entirely-aromatic liquid crystalline polymers.
However, such polymers are reused seldom. The polymers after disposal as waste materials will remain in the environment to impose heavy burden to the global environment. Waste materials from activities of industries and living are becoming serious as the social problems because of shortage of dumping sites, undesirable generation of dioxins on incineration, increase in carbon dioxide concentration in the air, and so forth. At the moment, development of materials and products in consideration of the global environment is being expected in connection with the carbon dioxide gas emission quotas, and waste materials. It is considered to be necessary to develop a technique which minimizes the consumption of the global resources to maintain the global environment.
In recent years, to meet the above problems, techniques have been developed for reuse of polymeric materials: for example, reuse technique such as reuse of used plastic parts after simple washing, and reworking of used resins for other uses of different added value; a material recycling technique such as molding of a used resin and a virgin resin in a sandwich state as described Japanese Patent Publication No. 6-24739; chemical recycling techniques such as decomposition of a used resin into a monomer after cutting into pieces; and thermal recycling technique such as use of waste resin as a fuel.
However, of such material recycling techniques, the reuse technique is limited to the use as the same parts. The material recycling technique has problems as to the stability of the properties owing to deterioration of the material, guarantee of the products, deterioration of appearance, and so forth. The use thereof is limited to lower grade articles practically. The chemical recycling technique is limited in the kinds of applicable materials, involving the problems of the monomer yield and a large amount of energy required for decomposition into the monomer. The thermal recycling technique has problems of the combustion heat inherent to the material, and reduction of carbon dioxide generation. Thus no recycling technique meets the requirement of the market. To meet these requirements, technical development of a novel resin which can be regenerated with a low energy, and a novel technique for regenerating the resin is wanted.
The resins are classified roughly into a condensation polymerization type polymers and an addition polymerization type polymers. The condensation polymerization type polymers typified by polyamides are readily depolymerized at the condensation sites by an acid or base, whereas the addition polymerization type polymers such as polystyrene requires a large amount of energy for depolymeization in an inert gas atmosphere under a high temperature.
Moreover, the decomposition products contain a mixture of dimer, trimer, tetramer, and so forth besides the monomer as described by T.Sawaguchi et al., Polym. Int. 49, 921 (2000). From the mixture, only the monomer which is polymerizable should be isolated, and a large amount of energy is necessary for the isolation. The yield of the recovered monomer is also important. Some of the addition polymerization polymers such as polypropylene cannot readily be depolymerized by the above-mentioned method.
Besides, decomposition of plastics using water or carbon dioxide in a supercritical state of a high temperature and a high pressure is investigated as described in Japanese Patent Laid-Open No. 8-72058. Such a method is not regarded to be the best method from the standpoint of the large-scale treatment and the large amount of energy to be inputted. Therefore, universally applicable techniques are demanded.
At the moment, the materials and products are demanded which meet the global environmental problem, such as carbon dioxide gas emission quotas and waste problems. On the other hand, minimization of consumption is required to maintain the natural resources.
The conventional addition type polymers which are synthesized by a monomer addition reaction can be regenerated for the material circulation only by chemical decomposition into the monomer.
Otherwise, a polymer can be formed from lower polymer molecules shorter in length than the practical polymer molecules as a kind of chemical parts (hereinafter the lower polymer being referred to as a “polymer” occasionally) by introducing a functional group into the parts for linkage-and-decomposition, and synthesizing the polymer from this parts. The polymer after use as an article like a molded product can be returned to the original chemical parts by breaking the linkage between the parts. The recovered parts can be formed again into a polymer by linking the parts together.
Specifically, a polymer (polymer) which has two condensable functional groups, and a molecule which has two functional groups capable of linking with the above condensable functional groups to serve as a coupler for the polymer are employed.
An example is explained below by taking a styrene polymer. A styrene polymer having a carboxyl group on each end of the molecule is employed as the two-functional polymer, and butanediol is employed as the coupling molecule having two functional groups linkable with the above functional groups of the polymer. These compounds are linked together by dehydration condensation in the presence of an acid catalyst to form a high polymer having a structure of successive linkage of the styrene polymer and the butanediol. Each of the styrene polymer moieties and butandiol moieties are linked by ester linkage.
The ester linkage can be broken by hydrolysis reaction into the original styrene polymer having a carboxyl group at the respective ends and butanediol. Thus the reaction is reversible. The compounds thus obtained can be converted into a high molecular styrene polymer by the same polycondensation reaction as above repeatedly without limitation of the repetition time basically.